Conventionally, as a medical diagnostic-imaging apparatus that can perform diagnosis of a function of a living body tissue of a subject, a positron emission computed tomography (PET) apparatus, a single photon emission computed tomography (SPECT) apparatus, and the like have been known.
A detector module, which is a gamma ray detector, in a PET apparatus receives, by a photomultiplier tube (PMT), a scintillation photon that is emitted when a gamma ray radiated from a subject enters a scintillator, and converts it into an electrical signal. For example, one scintillation photon that collides against a photoelectric surface of the photomultiplier tube emits one photon by the photoelectric effect. In a stage subsequent to the photoelectric surface, multiple dynodes that are positively charged are arranged. To these multiple dynodes, the photon accelerated by electric attraction travels, and collides with the dynodes. When the photon collides with the dynodes, several millions of photons are output as electrical signals from the photomultiplier tube. That is, a single photon put out from a single scintillation photon is multiplied to several millions of photons to flow as electrical signals. The multiplication factor of the number of photons at this time is called gain factor. This gain factor varies among photomultiplier tubes. For example, variations in the gain factor can range several-fold.
Accordingly, output currents vary according to a photomultiplier tube even if the same number of scintillation photons enter the photomultiplier tubes. Therefore, to express energy of an incident gamma ray by the scintillation photon, it is necessary to perform calibration, namely, energy calibration to make the output of the photomultiplier tubes uniform by amplifier circuitry in a later stage.
Therefore, it has generally been practiced that gamma rays are input to a detector module from a shielded discrete radio source or a shielded linear radio source by using a shielded discrete radio source or a shielded linear radio source of 68 Ge, or the like that radiates little-scattering monochromatic gamma rays with small dispersion, and a histogram of a time integral value of an output signal is generated to adjust the amplification factor in the amplifier circuitry so that peak positions are substantially consistent in all of the photomultiplier tubes.